The present invention relates to an image encoder, an image decoder and an image transmission device, particularly, to those suitably applicable to image transmission involving encoding and decoding of a monochrome image of a high bit depth.
Image data processed in widely used image encoders and decoders and in image transmission devices making use of such image encoders and decoders is, in many cases, of 8-bit precision per pixel. On the other hand, there are cases in which monochrome image information used, for example, in medical applications, monitoring cameras, and vehicle-mounted distance sensors requires higher-bit precision. Such monochrome images are referred to as high bit-depth monochrome images.
In Japanese Unexamined Patent Application Publication No. 2004-15226, an image encoder and an image decoder are disclosed which, having a simple configuration, process high-gradation image data (high bit-depth image data). In the image encoder and the image decoder, high-gradation image data is processed using basic image encoding means and basic image decoding means for processing 8-bit image data. In the image encoder disclosed in the above patent literature, a bit plane comprised of high-order 8 bits of an original high-gradation image is encoded, as a basic image, by basic image encoding means. In the image encoder, the output of the encoded image is decoded using basic image decoding means. The image data obtained by internally performed decoding is shifted left by a required number of bits, then the difference between the original image and the left-shifted image data is encoded again by the basic image encoding means. In this way, high-gradation image data can be processed using the basic image encoding means and basic image decoding means for processing 8-bit image data.